Reference is now made to FIG. 5 which shows a conventional vibration detecting device 101 of the type wherein a metal-made bush 1 has a bore 13 for a mounting bolt, and a surface which has an effective surface 1a with a diameter of the dimension .phi..sub.s, the bush being adapted to rest on an internal combustion engine. Held in the space defined between the bush and a case 3 is a Vibration-Voltage converter 2 which consists of a weight 8 and a piezoelectric element 5 disposed on a plate 4 which, in turn, rests on a fundamental surface 1b. An output signal from the piezoelectric element 5 is led from a terminal 6 to an output pin 11. An insulating sheet 7 is located on the upper surface of the terminal 6. The converter 2 is secured by way of a nut 9 screwed onto the threaded portion 1c of the bush 1. The bush 1 and the case 3 collaborate in forming an outer housing with the space therein plugged with a filling 10 consisting of an epoxy resin. An insulating tube 12 is provided around the periphery of the cylindrical portion of the bush 1.
As shown in FIG. 6, the vibration detecting device 101 of the above described type is secured to a boss 21 of an internal combustion engine by way of a bolt 14 in such a manner that the effective contact surface 1a is in face contact with the mounting surface of the boss. Any vibrations showing up in response to the running conditions of an internal combustion engine are transmitted through the effective contact surface to the device and the inertial force of the weight 8 and the result is then imposed on the piezoeletric element 5. The resulting signals from the latter are processed in order to distinguish particular signals (knock signals) that appear upon the occurence of knocking, which is a known phenomenon indicating lack of smooth functioning of an internal combustion engine. Based on the result of this detection, the performance parameters of the internal combustion engine are so controlled as to ensure an optimum output from the engine with maximum fuel efficiency.
It is a basic requirement for efficient functioning of the above device that the vibration of an engine is detected exactly. In order to achieve such precise detection, the condition in which the device is mounted on an internal combustion engine has to be optimum relative to its vibration detection characteristic. However, since the conventional device of the above described type has, as viewed in FIG. 6, an effective contact surface which has a greater diameter .phi..sub.s than a diameter .phi..sub.1 of the surface of the boss on which it is mounted (that is, .phi..sub.1 &lt;.phi..sub.s), the vibration detection characteristic of the device exhibits fluctuation as viewed in FIG. 7 even though it originally has a flat frequency characteristic. The greater the diameter .phi..sub.s is relative to the diameter .phi..sub.1, the more conspicuous is and the greater is the magnitude of the fluctuation of the vibration detection characteristic, especially in a low frequency region. In that situation, the result of the detection conducted by the device no longer truly represents the vibration of the engine in its actual state.